Extruder die head

ABSTRACT

An extruder die head, preferably a blown film head, comprises an internal cylindrical mandrel and a jacket, which envelops concentrically said mandrel. Between said mandrel and jacket an annular channel is formed that empties into a die slit. At least one melt-supplying line empties into the annular channel in the area opposite the die slit. To avoid slits between the central mandrel and the jacket, enveloping said mandrel, where the polymer melt can accumulate and deposit, the mandrel is made as one piece with a flange-like foot and is provided in its area, adjacent to the foot, with at least one borehole, which feeds in the melt. On the mandrel adjacent to the foot is shrunk a ring, which seals the slit between both. On the ring is mounted a cylindrical jacket and is connected sealingly to said jacket, which envelops the mandrel, forming an annular channel.

[0001] The invention relates to an extruder die head, preferably a blownfilm head, comprising an internal cylindrical mandrel and a jacket,which envelops concentrically said mandrel. Between said mandrel andjacket an annular channel is formed that empties into a die slit. Theinvention also comprises at least one line, which empties into theannular channel in the area opposite the die slit and which feeds amelt.

[0002] Blown film heads of this type exist. Usually the mandrel exhibitsone or several helical groove(s), whose depth decreases in the directionof the die slit, starting from the feed channel(s) so that the polymermelt overflows more and more the webs, which define the channels, andassumes a uniform flow in the axial direction. The prior art extruderdie heads exhibit the special problem that the mandrel on the other sideof the beginning of the annular channel is fitted sealingly into acylindrical borehole of the jacket. If, however, the extruder die headbecomes warm due to the polymer melt, flowing through said extruder diehead, the jacket expands more, on account of its larger diameter, thanthe central mandrel so that between the two of them a slit can form,into which penetrates the polymer melt that is fed in under pressure.Since the melt can accumulate in the area of this slit and because ofits long residence time in the hot die head, where it can decompose andbecome brittle, particles of the deposited melt can be dragged along bythe melt. Said particles become obvious in the extruded film tube or theinflated film bubble as the defective spots.

[0003] Therefore, the object of the invention is to provide an extruderdie head of the class described in the introductory part, wherein slits,in which the polymer melt can accumulate, deposit and become brittle,are avoided between the central mandrel and the jacket, enveloping saidmandrel.

[0004] The invention solves this problem in that the mandrel is made asone piece with a flange-like foot and is provided in its area, adjacentto the foot, with at least one borehole, which feeds in the melt, andthat on the mandrel adjacent to the foot is shrunk a ring, which sealsthe slit between both, and that on the ring is mounted a cylindricaljacket and is connected sealingly to said jacket, which envelops themandrel, forming an annular channel.

[0005] In the case of the extruder die head of the invention, themandrel is reliably sealed with respect to the flange-like foot, whichcan be connected to the mandrel as one material piece, by means of thering that is shrunk on. The ring is shrunk on the mandrel with such apretension that even if the mandrel and the ring become warm, no slit,into which the melt can penetrate and become brittle in said slit, canform between the two. The cylindrical jacket is mounted on the ring andconnected sealingly to it.

[0006] A preferred embodiment provides that the mandrel exhibits in itsarea adjacent to the foot one or several helical groove(s), whichexhibit(s) a depth that decreases in the direction of the die slit andwhich is (are) closed in the direction of the foot. The boreholes, whichfeed in the melt, empty in the initial regions of the grooves in thevicinity of the foot. In this respect the shrunk-on ring enclosessealingly the groove-free end of the mandrel and its initial region thatis provided with the grooves. In this embodiment the shrunk-on ringseals without a gap the groove edges in the foot region of the mandrelso that from the grooves the melt, fed into the initial region of thegrooves, can enter into the annular space between the mandrel and thejacket. In this space the melt can assume increasingly and uniformly anaxial flow.

[0007] Preferably the foot and the ring are provided with radial annularsurfaces, which lie in a common plane and which can lie sealingly oneach other and can be mounted together so as to seal.

[0008] Preferably the ring and the jacket are also provided with annularsurfaces, which lie in a common radial plane and which lie sealinglyside by side and are mounted together.

[0009] To fasten the jacket to the ring, the jacket can be mounted byintercalating the ring through an expansion screw connection with theannular surface of the foot.

[0010] If the extruder die head is a blown film head, boreholes orchannels can be provided in the known manner between the channels,feeding in the melt, in order to feed in and exhaust in the known mannerthe blowing air.

[0011] One embodiment of the invention is explained in detail below withreference to the drawing. The single figure is a schematic of anextruder die head with a cut ring, which is shrunk on the mandrel, and acut jacket.

[0012] The blown film head 1 comprises an internal mandrel 2, which ismade as one piece with a flange-shaped foot 3. Starting from its regionin the vicinity of the foot, the mandrel 2 is provided with multiplerows of helical grooves 4, whose depth decreases in the direction of thedie slit 5. The initial regions of the grooves 4 are closed in thedirection of the flange-like foot 3. Into the initial regions of thegrooves 3 empty the boreholes 6, through which the melt is fed in overthe non-illustrated boreholes and channels. On the mandrel 2 is shrunksealingly a ring 7, whose cross section is rectangular and whose bottomannular surface rests flush on the radial annular surface 8 of the foot3. Said annular surface forms with the jacket of the mandrel 2 a rightangle step. The shrunk on ring 7 envelops the initial region of thehelical grooves 3 of the mandrel, and in particular in such a mannerthat it rests with considerable pretension sealingly on the shaded partof the mandrel, thus on the webs and edges, defining the grooves. Theregion of the mandrel 2, enveloped by the ring 7, is defined by thedashed line 9 in the direction of the die slit 5. Thus, at the upperedge of the ring 7, marked by the dashed line 9, the melt enters intothe ring slit 10. On the ring 7 is mounted a cylindrical annular jacket11. The annular channel or annular slit 10 is formed between said jacketand the mandrel 2, enveloped by said jacket. This annular slit 10empties in the illustrated manner into the die slit 5.

[0013] The cylindrical jacket 11 is provided on its bottom end with aradial annular sealing surface, which rests flush and sealingly on theupper annular surface 12 of the ring 7 that is shrunk on. Thecylindrical jacket 1 is provided with axial boreholes (not illustrated),which align with the boreholes of the ring 7. Into these boreholes areinserted tightening screws (not illustrated), which are screwed into thethreaded boreholes (not illustrated) in the bottom flange-like foot 3.

1. Extruder die head (1), preferably a blown film head, comprising aninternal cylindrical mandrel (2) and a jacket (11), which envelopsconcentrically said mandrel, between said mandrel and jacket an annularchannel (10) being formed that empties into a die slit (5), andcomprising at least one line (6), which empties into the annular channel(10) in the area opposite the die slit and which feeds in a melt,characterized in that the mandrel (2) is made as one piece with aflange-like foot (3) and is provided in its area, adjacent to the foot,with at least one borehole (6), which feeds in the melt, and that on themandrel (2) adjacent to the foot (3) is shrunk a ring (7), which sealsthe slit between both, and that on the ring (7) is mounted a cylindricaljacket and is connected sealingly to said jacket, which envelops themandrel (2), forming an annular channel (10).
 2. Extruder die head, asclaimed in claim 1 , characterized in that the mandrel (2) exhibits inits area adjacent to the foot (3) one or several helical groove(s) (4)which exhibit(s) a depth that decreases in the direction of the die slit(5) and which is (are) closed in the direction of the foot (3) and intheir initial regions in the vicinity of the foot empty the boreholes(6), which feed in the melt, and that the shrunk-on ring (7) enclosessealingly the groove-free end of the mandrel and its initial region thatis provided with the grooves (4).
 3. Extruder die head, as claimed inclaim 1 or 2 , characterized in that the foot (3) and the ring (7) areprovided with radial annular surfaces, which lie in a common plane. 4.Extruder die head, as claimed in any one of the claims 1 to 3 ,characterized in that the ring (7) and the jacket (11) are provided withannular surfaces, which lie in a common radial plane.
 5. Extruder diehead, as claimed in any one of the claims 1 to 4 , characterized in thatthe jacket (11) is fixed in position with the ring (7).
 6. Extruder diehead, as claimed in any one of the claims 1 to 5 , characterized in thatthe jacket (11) is mounted by intercalating the ring (7) throughexpansion screw connection with the annular surface (8) of the foot (3).